1. Field of the Invention
The present invention relates to a circuit arrangement for an ultrasonic burglar security system operating in accordance with the Doppler principle, and more particularly to such an arrangement which comprises a transmitting transducer which is fed with an ac voltage from an oscillator and which continuously transmits ultrasonic radiation during operation, a receiving transducer and a receiving circuit connected to the receiving transducer and which has an analysis circuit and a phase detector which continuously compares the received signal of the receiving transducer with the ac voltage to form a detection signal which is dependent upon the mutual phase angle and which is fed to the analysis circuit, the analysis circuit responding to the ac component of the detection signal.
2. Description of the Prior Art
The German published application No. 2,613,375 of Oct. 13, 1977, discloses a circuit arrangement of the type generally set forth above. The detector of the circuit arrangement has an amplitude limiter of a type known per se for frequency modulation, which is provided for the received signal and by which the entire amplitude modulation of the received signal is cut off, for which purpose the received signal is previously amplified to such an extent that the signal, which has been entirely cut off in amplitude, still has adequate intensity for further processing. A product detector is supplied with the signal having the limited amplitude and with a reference signal derived from the transmitter, and supplies a frequency difference signal which is available at the output terminal for feeding a device which triggers an alarm.
In special applications, in an ultrasonic burglar security system as described in the aforementioned German published application, it can be necessary to select the exact location at which, in particular, the ultrasonic receiving transducer is set up (or that part of the device containing the receiving transducer) in order to insure that, due to interferences, the ultrasonic transducer does not receive a signal which, for the phase comparison with the signal fed from the oscillator to the phase detector, possesses a phase state which is unfavorable for analysis. This problem can be overcome in a simple manner by a relatively slight, differential change of location of the receiving transducer and/or the transmitting transducer. On the other hand, however, it could also be necessary, in the event of a change in the set-up of the area to be monitored, for example in the event that a cupboard or the like is moved, to carry out a location correction of at least one transducer (or possibly of the transmitting transducer).